That's it, boys and girls. The show's over when it comes to watching YouTube videos through the Wii U browser.
All the way back in February, YouTube shared a notice on the Wii U browser to let people know that service was coming to an end. It took 4+ months, but that day has finally come. Now if you try to view YouTube videos through the Wii U browser, you get a page that can't load anything.
If you want to do any YouTube browsing from here on out, you can still use the YouTube app on Wii U. No word on how long that app will be supported.